A typical radio-frequency (RF) communications system consists of a full duplex base station and a plurality of half duplex radios. Information is typically transferred between the radios and the base station on a single RF channel consisting of a duplex pair of frequencies. The first frequency or the forward channel is used by the radios to transmit information to the base, and the second frequency or the reverse channel is used exclusively by the base to transmit information to the radios. Because there is a plurality of radios and only one forward channel, transmission time on the forward channel must be shared by the various radios. This channel sharing creates a potential for conflict among the radios as the radios compete for transmission time. To resolve the possible conflicts (known as collisions) and to coordinate the exchange of information between the radios and the base, a multiple access protocol is implemented. A protocol which is commonly used is "slotted Aloha".
Slotted Aloha divides time on the forward channel into periods, and at the start of each period, the base announces a selected number of fixed length time slots during which the half duplex radios may transmit information frames on the forward channel. Upon receipt of the fixed slot information, each radio having an information frame to transmit randomly selects one of the announced time slots, and transmits an information frame during the selected time slot. In such a system, three possible events may occur during each time slot. First, because the time slot selection process is random, a slot may be left unselected and wasted. Second, a single information frame may be received during the time slot, in which case, the information is successfully received by the base. Third, a collision occurs. In such an instance, all of the information frames received are disregarded by the base and the sending radios must transmit the information again during the next period.
Slotted Aloha has a number of significant drawbacks. First of all, slotted Aloha suffers from low utilization. It is generally accepted that for slotted Alhoa, only about 35% of the forward channel time is actually used for successful transmissions. The rest of the channel time is wasted on collisions and unselected slots. Second, slotted Aloha becomes unstable under heavy channel loading. Above a utilization rate of about 35%, the efficiency of slotted Aloha begins to drop rapidly. This means that during peak traffic times when successful transmissions are needed the most, slotted Aloha's efficiency is at its worst. Third, slotted Aloha coordinates the exchange of information between the base station and the radios by assigning fixed length time slots. These time slots are fixed in length regardless of the needs of the radios. Therefore, if a radio has a short information frame requiring only half of a time slot to transmit, the remaining half of the time slot is wasted. Thus, the slotted Aloha protocol inherently wastes a large amount of forward channel capacity.
Other methods such as carrier sensing and polling have been implemented to try to improve efficiency, but these methods have not produced much better results. Therefore, a need exists for a more efficient method and apparatus for coordinating the transfer of information between a plurality of radios and a base station.